1. Field of the Invention
The present invention relates to an image forming apparatus and a control method thereof, a computer program, and a computer readable storage medium.
2. Description of the Related Art
A VOD (Video On Demand) streaming system and a network monitoring system have been known as an apparatus for displaying/reproducing remote moving image content via a network. In the VOD streaming system, content stored on a remote server is viewed on a personal computer connected to a network. In the network monitoring system, an image taken by a remote monitoring camera is displayed on a personal computer or a dedicated monitor connected to a network for monitoring.
These systems are constructed by connecting a storage server or camera server with a personal computer or display terminal apparatus via a network. In order to view the image on a personal computer, various kinds of software such as an application for viewing content are needed. Image data is compressed using formats such as MPEG-2 and MPEG-4 to reduce data quantities and is packetized for transmission. The RTP protocol is used in most cases as a transmission protocol to reduce communication bandwidth and simplify control.
When the RTP protocol is used for transmission, a data loss on a network is a problem. Due to abnormal routing paths or disturbances in a network, packet losses, delays, or changed order of arriving packets may occur. This causes phenomena such as a sudden stop of moving images and a deformation of part of a screen, leading to significant degradation of reproducing/displaying quality.
Countermeasures against data losses during transmission of image data using the RTP protocol are given as shown below:
(1) First, a method of combining with a retransmission protocol is available. By a combined use of the RTP protocol with a retransmission protocol such as TCP/IP, a re-transmission demand is made using the re-transmission protocol when a data loss occurs. A display control operation is performed after performing buffering to wait for arrival of re-transmitted data in an input stage on a receiving apparatus side (See, for example, Japanese Patent Application Laid-Open No. 2003-169040).
(2) A method of increasing error resilience of the RTP protocol is available. This method is generally named FEC (Forward Error Correction). By adding a redundant code on a transmitting side in advance, data can be restored on a receiving side even if a packet loss occurs during transmission. Redundant encoding using Tornado code, LT code, or Reed-Solomon code is well known. A group of n packets is formed and, based on the group, m packets are generated (n<m). If a packet loss occurs, content of the lost packet can be restored using other packets in the group (See, for example, Japanese Patent Application Laid-Open No. 2001-045098).
Methods shown below are provided as examples of performing a more fine-tuned control operation.
(3) A method of selecting an error resilience method based on information about a congestion state of a network, terminal load, buffering state, or user instruction, has been devised. A statistical information observation unit collects the above information and switches an algorithm of error resilience, particularly FEC and ARQ (See, for example, Japanese Patent Application Laid-Open No. 2002-141964).
(4) A method of receiving and reproducing image data by specifying one of a plurality of image data with various error resilience, has been devised. A plurality of video streams including I frames with different periodicity are stored on a server. After specifying one video stream using an instruction signal from a receiving terminal, the pertinent stream is delivered (See, for example, Japanese Patent Application Laid-Open No. 2003-032690).
(5) A method of dividing an image into areas and transmitting each area with a changed resolution of hierarchical encoding, has been devised. Image data is divided into areas and each area is coded hierarchically in a base layer and an enhancement layer. At the time of transmission, first, the whole area is transmitted with a resolution of the base layer and then enhancement layer data of important areas is transmitted within a range permitted by the communication bands (See, for example, Japanese Patent Application Laid-Open No. 2004-096309).
In the above conventional examples, however, there is a problem in that image quality and real-time are not compatible with each other. In the conventional examples (1) and (3), a large amount of buffering is needed to ensure an arrival wait time of are transmitted packet and thus, there is a disadvantage of significantly impairing real-time characteristic of display. In the conventional example (2), there is a problem in that, since a bandwidth in proportion to the error resilience strength is required, image quality is degraded if the error resilience is enhanced when the bandwidth remains constant. For example, while losses or screen deformations in units of lines or frames occur rarely, there are cases in which block noise, mosquito noise, or the like increases. In the conventional example (4), there is a problem in that image quality is uniformly determined by the type of stream provided by the server and there is no room for further improvement of image quality. In the conventional example (5), there is a problem in that the method is applicable only to hierarchically coded data.